Despite recent advances in understanding of the many aspects of the pathogenesis of acute lung injury (ALI) and its severe form acute respiratory distress syndrome (ARDS), there are currently no effective pharmacological or cell-based treatments for the disease. Employing novel animal models and integrated pharmacological and molecular and cellular biological approaches, the focus of Project 3 in the context of the Program Project will be to define for the first time the reprogramming of lung macrophages (M?) to the M2 lineage as regulated by the interaction of endothelial cell (EC)-expressed Jagged1 (Jag1, a Notch ligand) with M?- expressed Notch. We will determine whether these ?instructed? cells have the potential to resolve inflammatory lung injury. We hope through these studies to develop novel therapeutics to promote resolution of inflammatory lung injury and enhance lung?s tolerance to injury. Our Supporting Data show that disruption of Jag1 in EC induces reprogramming of M? towards the M2-like anti-inflammatory phenotype and resolution of lung inflammation. Thus, in Project 3, we will test the hypothesis that the interaction of M? with lung EC-expressed Jag1 mediates Notch signaling in M? and thereby determines their polarity and their ability to resolve inflammatory lung injury. The proposed studies will address the following Specific Aims. In Aim 1, we will determine the regulation of polarization of M? by EC-expressed Jag1 and its role in resolving inflammatory lung injury and promoting tolerance to injury induced by secondary infection. In Aim 2, we will define the signaling mechanisms activated in M? that mediate EC Jag1-induced M? polarization and the therapeutic potential of strategies of specifically targeting Jag1 in resolving inflammatory lung injury. Upon completion of these studies, we will identify not only the fundamental role of lung EC Jag1 in mediating M? polarization via interaction with Notch signaling and its significance in resolving inflammatory lung injury but also novel therapeutic approaches and targets for the treatment of ALI/ARDS.